According to a known technique, known as WILHELMY technique, the surface tension of a static liquid is measured by means of a very thin and rectangular platinum blade passing through the surface to be studied. The platinum blade is suspended at one end of a wire and brought into contact of the liquid surface. FIG. 1 to which is now referred to, schematically illustrates a device carrying out the WILHELMY technique. The device comprises a platinum blade 1, the dimensions of which, by way of example, can be of 2 cm wide, 1 cm high and 0.01 cm thick. This blade is coupled to a balance 2 or a strain gauge by means of a wire 3.
Let us sum up briefly the theory which governs the operation of such a surface tension sensor. When a perfectly wettable platinum blade 1 is dipped into a liquid 4, this blade is subjected to capillary forces S and to buoyancy P.sub.A. The buoyancy corresponds to the volume immerged multiplied by the density. In the case of a conventional surface tension measuring of a static liquid, the immersion height of the blade is null; the P.sub.A force is thus null. In order to extract this blade out of the liquid, a vertical outwardly oriented force f must be exerted, proportional to the blade perimeter and to the liquid surface tension .sigma.. This last parameter depends on the liquid, and is constant for a pure liquid, for a given temperature. For surfactant solutions, this parameter depends on the concentration and the age of the surface.
The equation governing the equilibrium of the forces is the following: EQU F=S-P.sub.A ( 1) EQU with S=.sigma..L.cos .theta.=m.g (2)
wherein:
.theta. is the liquid/solid wetting angle. For a perfectly wettable sensor, the solid/liquid wetting angle is equal to zero. So, we have cos .theta.=1. PA1 m is the compensation mass of the desequilibrium in grams PA1 g is the acceleration of gravity: 9.81.10.sup.-3 N.s.sup.-2. PA1 .sigma. is the surface tension in dyn.cm.sup.-1. PA1 L is the length in cm whereon the meniscus is caught, i.e., the perimeter of the blade portion in contact with the liquid.
Thus, we have: ##EQU1##
This technique is perfectly satisfactory when it deals with the surface tension measuring of a static liquid but raises problems for the surface tension measuring of a flow on a horizontal or slightly inclined plane, the measures being in fact erroneous due to the drag force acting on the blade. This was demonstrated in an Article of R. DEFAY and J. HOMMELEN published in the journal de l'industrie Belge (23, Jun. 1958, pages 597-614). On page 602, paragraph 6, it is mentioned that the ascending and drag forces have always been too important to be able to provide for an even approximative measure of the surface tension.
According to another known method which raises the same problems, the rectangular blade was replaced by a ring.
Thus, it is an object of the present invention to provide a device allowing an experimenter to measure the surface tension of a liquid, particularly adapted to a flow on a horizontal or slightly inclined plane.